Sequence Boundaries in Terrestrial Foreland-Basin Strata: Do They Lie Above or Below the Amalgamated Fluvial Facies Tract?

Timothy F. Lawton¹ and Amy E. Christensen^2
1 Institute of Tectonic Studies, New Mexico State University, Las Cruces, NM
² New Mexico State University, Las Cruces, NM

Models for non-marine sequence stratigraphy in foreland basins typically place the sequence boundary, the fundamental break that begins the succeeding stratigraphic cycle, beneath the amalgamated fluvial facies tract, a succession of sand-rich, braided-fluvial deposits. Sandstone amalgamation is inferred to result from low accommodation caused by base-level fall at the downstream end of the fluvial system. Nevertheless, several data sets from the Drip Tank Member of the Upper Cretaceous Straight Cliffs Formation in southern Utah, USA, indicate that the sequence boundary in fact lies above this archetypical amalgamated fluvial sandstone. Four observations indicate a gradual upsection change in fluvial style from underlying mudstone-rich fluvial strata, through the Drip Tank, to an abrupt change at its top: (1) Channel depth, recorded by preserved story thickness, decreases gradually upsection to the top of the Drip Tank. (2) The abundance of lateral-accretion bedding likewise decreases. (3) Paleocurrent indicators record a progressive swing from northeast- to southeast-flowing river systems. (4) Sandstone composition changes in concert with paleocurrents, with northeast-flowing rivers carrying feldspathic detritus and southeast-flowing rivers transporting quartzose detritus. The top of the Drip Tank is locally truncated beneath broad valley-like features incompletely filled with quartzose sandstone and Drip Tank intraclasts. We interpret the paleovalleys as a sediment-bypass system overlying a sequence boundary above the amalgamated sandstone. Feldspathic sandstone overlies the paleovalley fill and marks a return to fluvial deposition by northeast-draining rivers. Although amalgamated fluvial sandstones likely overlie sequence boundaries elsewhere in this foreland basin, the Drip Tank does not. We infer that an increased fluvial depositional slope, rather than base-level shift, is the dominant factor influencing sequence architecture and position of the sequence boundary.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005